. /include/boost/corosio /native /detail /select /select_scheduler.hpp
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1   // 1   //
2   // Copyright (c) 2026 Steve Gerbino 2   // Copyright (c) 2026 Steve Gerbino
3   // Copyright (c) 2026 Michael Vandeberg 3   // Copyright (c) 2026 Michael Vandeberg
4   // 4   //
5   // Distributed under the Boost Software License, Version 1.0. (See accompanying 5   // Distributed under the Boost Software License, Version 1.0. (See accompanying
6   // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) 6   // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
7   // 7   //
8   // Official repository: https://github.com/cppalliance/corosio 8   // Official repository: https://github.com/cppalliance/corosio
9   // 9   //
10   10  
11   #ifndef BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP 11   #ifndef BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP
12   #define BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP 12   #define BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP
13   13  
14   #include <boost/corosio/detail/platform.hpp> 14   #include <boost/corosio/detail/platform.hpp>
15   15  
16   #if BOOST_COROSIO_HAS_SELECT 16   #if BOOST_COROSIO_HAS_SELECT
17   17  
18   #include <boost/corosio/detail/config.hpp> 18   #include <boost/corosio/detail/config.hpp>
19   #include <boost/capy/ex/execution_context.hpp> 19   #include <boost/capy/ex/execution_context.hpp>
20   20  
21   #include <boost/corosio/native/detail/reactor/reactor_scheduler.hpp> 21   #include <boost/corosio/native/detail/reactor/reactor_scheduler.hpp>
22   #include <boost/corosio/native/detail/reactor/reactor_signal_pipe.hpp> 22   #include <boost/corosio/native/detail/reactor/reactor_signal_pipe.hpp>
23   23  
24   #include <boost/corosio/native/detail/select/select_traits.hpp> 24   #include <boost/corosio/native/detail/select/select_traits.hpp>
25   #include <boost/corosio/detail/timer_service.hpp> 25   #include <boost/corosio/detail/timer_service.hpp>
26   #include <boost/corosio/native/detail/make_err.hpp> 26   #include <boost/corosio/native/detail/make_err.hpp>
27   27  
28   #include <boost/corosio/detail/except.hpp> 28   #include <boost/corosio/detail/except.hpp>
29   29  
30   #include <sys/select.h> 30   #include <sys/select.h>
31   #include <unistd.h> 31   #include <unistd.h>
32   #include <errno.h> 32   #include <errno.h>
33   #include <fcntl.h> 33   #include <fcntl.h>
34   34  
35   #include <atomic> 35   #include <atomic>
36   #include <chrono> 36   #include <chrono>
37   #include <cstdint> 37   #include <cstdint>
38   #include <limits> 38   #include <limits>
39   #include <mutex> 39   #include <mutex>
40   #include <new> 40   #include <new>
41   #include <unordered_map> 41   #include <unordered_map>
42   42  
43   namespace boost::corosio::detail { 43   namespace boost::corosio::detail {
44   44  
45   struct select_op; 45   struct select_op;
46   46  
47   /** POSIX scheduler using select() for I/O multiplexing. 47   /** POSIX scheduler using select() for I/O multiplexing.
48   48  
49   This scheduler implements the scheduler interface using the POSIX select() 49   This scheduler implements the scheduler interface using the POSIX select()
50   call for I/O event notification. It inherits the shared reactor threading 50   call for I/O event notification. It inherits the shared reactor threading
51   model from reactor_scheduler: signal state machine, inline completion 51   model from reactor_scheduler: signal state machine, inline completion
52   budget, work counting, and the do_one event loop. 52   budget, work counting, and the do_one event loop.
53   53  
54   The design mirrors epoll_scheduler for behavioral consistency: 54   The design mirrors epoll_scheduler for behavioral consistency:
55   - Same single-reactor thread coordination model 55   - Same single-reactor thread coordination model
56   - Same deferred I/O pattern (reactor marks ready; workers do I/O) 56   - Same deferred I/O pattern (reactor marks ready; workers do I/O)
57   - Same timer integration pattern 57   - Same timer integration pattern
58   58  
59   Known Limitations: 59   Known Limitations:
60   - FD_SETSIZE (~1024) limits maximum concurrent connections 60   - FD_SETSIZE (~1024) limits maximum concurrent connections
61   - O(n) scanning: rebuilds fd_sets each iteration 61   - O(n) scanning: rebuilds fd_sets each iteration
62   - Level-triggered only (no edge-triggered mode) 62   - Level-triggered only (no edge-triggered mode)
63   63  
64   @par Thread Safety 64   @par Thread Safety
65   All public member functions are thread-safe. 65   All public member functions are thread-safe.
66   */ 66   */
67   class BOOST_COROSIO_DECL select_scheduler final : public reactor_scheduler 67   class BOOST_COROSIO_DECL select_scheduler final : public reactor_scheduler
68   { 68   {
69   public: 69   public:
70   /** Construct the scheduler. 70   /** Construct the scheduler.
71   71  
72   Creates a self-pipe for reactor interruption. 72   Creates a self-pipe for reactor interruption.
73   73  
74   @param ctx Reference to the owning execution_context. 74   @param ctx Reference to the owning execution_context.
75   @param concurrency_hint Hint for expected thread count (unused). 75   @param concurrency_hint Hint for expected thread count (unused).
76   */ 76   */
77   select_scheduler(capy::execution_context& ctx, int concurrency_hint = -1); 77   select_scheduler(capy::execution_context& ctx, int concurrency_hint = -1);
78   78  
79   /// Destroy the scheduler. 79   /// Destroy the scheduler.
80   ~select_scheduler() override; 80   ~select_scheduler() override;
81   81  
82   select_scheduler(select_scheduler const&) = delete; 82   select_scheduler(select_scheduler const&) = delete;
83   select_scheduler& operator=(select_scheduler const&) = delete; 83   select_scheduler& operator=(select_scheduler const&) = delete;
84   84  
85   /// Shut down the scheduler, draining pending operations. 85   /// Shut down the scheduler, draining pending operations.
86   void shutdown() override; 86   void shutdown() override;
87   87  
88   /** Return the maximum file descriptor value supported. 88   /** Return the maximum file descriptor value supported.
89   89  
90   Returns FD_SETSIZE - 1, the maximum fd value that can be 90   Returns FD_SETSIZE - 1, the maximum fd value that can be
91   monitored by select(). Operations with fd >= FD_SETSIZE 91   monitored by select(). Operations with fd >= FD_SETSIZE
92   will fail with EINVAL. 92   will fail with EINVAL.
93   93  
94   @return The maximum supported file descriptor value. 94   @return The maximum supported file descriptor value.
95   */ 95   */
96   static constexpr int max_fd() noexcept 96   static constexpr int max_fd() noexcept
97   { 97   {
98   return FD_SETSIZE - 1; 98   return FD_SETSIZE - 1;
99   } 99   }
100   100  
101   /** Register a descriptor for persistent monitoring. 101   /** Register a descriptor for persistent monitoring.
102   102  
103   The fd is added to the registered_descs_ map and will be 103   The fd is added to the registered_descs_ map and will be
104   included in subsequent select() calls. The reactor is 104   included in subsequent select() calls. The reactor is
105   interrupted so a blocked select() rebuilds its fd_sets. 105   interrupted so a blocked select() rebuilds its fd_sets.
106   106  
107   @param fd The file descriptor to register. 107   @param fd The file descriptor to register.
108   @param desc Pointer to descriptor state for this fd. 108   @param desc Pointer to descriptor state for this fd.
109   109  
110   @return The error if the fd cannot be tracked, otherwise a 110   @return The error if the fd cannot be tracked, otherwise a
111   default constructed error code. 111   default constructed error code.
112   */ 112   */
113   std::error_code 113   std::error_code
114   register_descriptor(int fd, reactor_descriptor_state* desc) const; 114   register_descriptor(int fd, reactor_descriptor_state* desc) const;
115   115  
116   /** Deregister a persistently registered descriptor. 116   /** Deregister a persistently registered descriptor.
117   117  
118   @param fd The file descriptor to deregister. 118   @param fd The file descriptor to deregister.
119   */ 119   */
120   void deregister_descriptor(int fd) const; 120   void deregister_descriptor(int fd) const;
121   121  
122   /** Interrupt the reactor so it rebuilds its fd_sets. 122   /** Interrupt the reactor so it rebuilds its fd_sets.
123   123  
124   Called when a write, connect, or write-wait op is registered 124   Called when a write, connect, or write-wait op is registered
125   after the reactor's snapshot was taken. Without this, 125   after the reactor's snapshot was taken. Without this,
126   select() may block not watching for writability on the fd. 126   select() may block not watching for writability on the fd.
127   */ 127   */
128   void notify_reactor() const; 128   void notify_reactor() const;
129   129  
130   /// Watch the read end of the POSIX signal self-pipe (see scheduler.hpp). 130   /// Watch the read end of the POSIX signal self-pipe (see scheduler.hpp).
HITCBC 131   61 [[nodiscard]] std::error_code register_signal_reader(int read_fd) override 131   61 [[nodiscard]] std::error_code register_signal_reader(int read_fd) override
132   { 132   {
HITCBC 133   61 return register_descriptor(read_fd, signal_pipe_reader_.arm()); 133   61 return register_descriptor(read_fd, signal_pipe_reader_.arm());
134   } 134   }
135   135  
136   private: 136   private:
137   void run_task(lock_type& lock, context_type& ctx, long timeout_us) override; 137   void run_task(lock_type& lock, context_type& ctx, long timeout_us) override;
138   void interrupt_reactor() const override; 138   void interrupt_reactor() const override;
139   long calculate_timeout(long requested_timeout_us) const; 139   long calculate_timeout(long requested_timeout_us) const;
140   140  
141   // Watches the global signal self-pipe's read end (armed lazily by 141   // Watches the global signal self-pipe's read end (armed lazily by
142   // register_signal_reader on the first signal registration). 142   // register_signal_reader on the first signal registration).
143   reactor_signal_pipe_reader signal_pipe_reader_; 143   reactor_signal_pipe_reader signal_pipe_reader_;
144   144  
145   // Self-pipe for interrupting select() 145   // Self-pipe for interrupting select()
146   int pipe_fds_[2]; // [0]=read, [1]=write 146   int pipe_fds_[2]; // [0]=read, [1]=write
147   147  
148   // Per-fd tracking for fd_set building 148   // Per-fd tracking for fd_set building
149   mutable std::unordered_map<int, reactor_descriptor_state*> 149   mutable std::unordered_map<int, reactor_descriptor_state*>
150   registered_descs_; 150   registered_descs_;
151   mutable int max_fd_ = -1; 151   mutable int max_fd_ = -1;
152   }; 152   };
153   153  
HITCBC 154   950 inline select_scheduler::select_scheduler(capy::execution_context& ctx, int) 154   950 inline select_scheduler::select_scheduler(capy::execution_context& ctx, int)
HITCBC 155   950 : pipe_fds_{-1, -1} 155   950 : pipe_fds_{-1, -1}
HITCBC 156   950 , max_fd_(-1) 156   950 , max_fd_(-1)
157   { 157   {
HITCBC 158   950 if (::pipe(pipe_fds_) < 0) 158   950 if (::pipe(pipe_fds_) < 0)
HITCBC 159   1 detail::throw_system_error(make_err(errno), "pipe"); 159   1 detail::throw_system_error(make_err(errno), "pipe");
160   160  
HITCBC 161   2838 for (int i = 0; i < 2; ++i) 161   2838 for (int i = 0; i < 2; ++i)
162   { 162   {
HITCBC 163   1895 int flags = ::fcntl(pipe_fds_[i], F_GETFL, 0); 163   1895 int flags = ::fcntl(pipe_fds_[i], F_GETFL, 0);
HITCBC 164   1895 if (flags == -1) 164   1895 if (flags == -1)
165   { 165   {
HITCBC 166   2 int errn = errno; 166   2 int errn = errno;
HITCBC 167   2 ::close(pipe_fds_[0]); 167   2 ::close(pipe_fds_[0]);
HITCBC 168   2 ::close(pipe_fds_[1]); 168   2 ::close(pipe_fds_[1]);
HITCBC 169   2 detail::throw_system_error(make_err(errn), "fcntl F_GETFL"); 169   2 detail::throw_system_error(make_err(errn), "fcntl F_GETFL");
170   } 170   }
HITCBC 171   1893 if (::fcntl(pipe_fds_[i], F_SETFL, flags | O_NONBLOCK) == -1) 171   1893 if (::fcntl(pipe_fds_[i], F_SETFL, flags | O_NONBLOCK) == -1)
172   { 172   {
HITCBC 173   2 int errn = errno; 173   2 int errn = errno;
HITCBC 174   2 ::close(pipe_fds_[0]); 174   2 ::close(pipe_fds_[0]);
HITCBC 175   2 ::close(pipe_fds_[1]); 175   2 ::close(pipe_fds_[1]);
HITCBC 176   2 detail::throw_system_error(make_err(errn), "fcntl F_SETFL"); 176   2 detail::throw_system_error(make_err(errn), "fcntl F_SETFL");
177   } 177   }
HITCBC 178   1891 if (::fcntl(pipe_fds_[i], F_SETFD, FD_CLOEXEC) == -1) 178   1891 if (::fcntl(pipe_fds_[i], F_SETFD, FD_CLOEXEC) == -1)
179   { 179   {
HITCBC 180   2 int errn = errno; 180   2 int errn = errno;
HITCBC 181   2 ::close(pipe_fds_[0]); 181   2 ::close(pipe_fds_[0]);
HITCBC 182   2 ::close(pipe_fds_[1]); 182   2 ::close(pipe_fds_[1]);
HITCBC 183   2 detail::throw_system_error(make_err(errn), "fcntl F_SETFD"); 183   2 detail::throw_system_error(make_err(errn), "fcntl F_SETFD");
184   } 184   }
185   } 185   }
186   186  
HITCBC 187   943 timer_svc_ = &get_timer_service(ctx, *this); 187   943 timer_svc_ = &get_timer_service(ctx, *this);
HITCBC 188   943 timer_svc_->set_on_earliest_changed( 188   943 timer_svc_->set_on_earliest_changed(
HITCBC 189   3630 timer_service::callback(this, [](void* p) { 189   3212 timer_service::callback(this, [](void* p) {
HITCBC 190   2687 static_cast<select_scheduler*>(p)->interrupt_reactor(); 190   2269 static_cast<select_scheduler*>(p)->interrupt_reactor();
HITCBC 191   2687 })); 191   2269 }));
192   192  
HITCBC 193   943 completed_ops_.push(&task_op_); 193   943 completed_ops_.push(&task_op_);
HITCBC 194   964 } 194   964 }
195   195  
HITCBC 196   1886 inline select_scheduler::~select_scheduler() 196   1886 inline select_scheduler::~select_scheduler()
197   { 197   {
HITCBC 198   943 if (pipe_fds_[0] >= 0) 198   943 if (pipe_fds_[0] >= 0)
HITCBC 199   943 ::close(pipe_fds_[0]); 199   943 ::close(pipe_fds_[0]);
HITCBC 200   943 if (pipe_fds_[1] >= 0) 200   943 if (pipe_fds_[1] >= 0)
HITCBC 201   943 ::close(pipe_fds_[1]); 201   943 ::close(pipe_fds_[1]);
HITCBC 202   1886 } 202   1886 }
203   203  
204   inline void 204   inline void
HITCBC 205   943 select_scheduler::shutdown() 205   943 select_scheduler::shutdown()
206   { 206   {
HITCBC 207   943 shutdown_drain(); 207   943 shutdown_drain();
208   208  
HITCBC 209   943 if (pipe_fds_[1] >= 0) 209   943 if (pipe_fds_[1] >= 0)
HITCBC 210   943 interrupt_reactor(); 210   943 interrupt_reactor();
HITCBC 211   943 } 211   943 }
212   212  
213   inline std::error_code 213   inline std::error_code
HITCBC 214   4813 select_scheduler::register_descriptor( 214   4162 select_scheduler::register_descriptor(
215   int fd, reactor_descriptor_state* desc) const 215   int fd, reactor_descriptor_state* desc) const
216   { 216   {
HITCBC 217   4813 if (fd < 0 || fd >= FD_SETSIZE) 217   4162 if (fd < 0 || fd >= FD_SETSIZE)
HITCBC 218   1 return make_err(EMFILE); 218   1 return make_err(EMFILE);
219   219  
HITCBC 220   4812 desc->registered_events = reactor_event_read | reactor_event_write; 220   4161 desc->registered_events = reactor_event_read | reactor_event_write;
HITCBC 221   4812 desc->fd = fd; 221   4161 desc->fd = fd;
HITCBC 222   4812 desc->scheduler_ = this; 222   4161 desc->scheduler_ = this;
HITCBC 223   4812 desc->mutex.set_enabled(reactor_io_locking_); 223   4161 desc->mutex.set_enabled(reactor_io_locking_);
HITCBC 224   4812 desc->ready_events_.store(0, std::memory_order_relaxed); 224   4161 desc->ready_events_.store(0, std::memory_order_relaxed);
225   225  
226   { 226   {
HITCBC 227   4812 conditionally_enabled_mutex::scoped_lock lock(desc->mutex); 227   4161 conditionally_enabled_mutex::scoped_lock lock(desc->mutex);
HITCBC 228   4812 desc->impl_ref_.reset(); 228   4161 desc->impl_ref_.reset();
HITCBC 229   4812 desc->read_ready = false; 229   4161 desc->read_ready = false;
HITCBC 230   4812 desc->write_ready = false; 230   4161 desc->write_ready = false;
HITCBC 231   4812 } 231   4161 }
232   232  
233   { 233   {
HITCBC 234   4812 mutex_type::scoped_lock lock(mutex_); 234   4161 mutex_type::scoped_lock lock(mutex_);
235   try 235   try
236   { 236   {
HITCBC 237   4812 registered_descs_[fd] = desc; 237   4161 registered_descs_[fd] = desc;
238   } 238   }
HITCBC 239   1 catch (std::bad_alloc const&) 239   1 catch (std::bad_alloc const&)
240   { 240   {
HITCBC 241   1 return make_err(ENOMEM); 241   1 return make_err(ENOMEM);
HITCBC 242   1 } 242   1 }
HITCBC 243   4811 if (fd > max_fd_) 243   4160 if (fd > max_fd_)
HITCBC 244   4761 max_fd_ = fd; 244   4110 max_fd_ = fd;
HITCBC 245   4812 } 245   4161 }
246   246  
HITCBC 247   4811 interrupt_reactor(); 247   4160 interrupt_reactor();
HITCBC 248   4811 return {}; 248   4160 return {};
249   } 249   }
250   250  
251   inline void 251   inline void
HITCBC 252   4751 select_scheduler::deregister_descriptor(int fd) const 252   4100 select_scheduler::deregister_descriptor(int fd) const
253   { 253   {
HITCBC 254   4751 mutex_type::scoped_lock lock(mutex_); 254   4100 mutex_type::scoped_lock lock(mutex_);
255   255  
HITCBC 256   4751 auto it = registered_descs_.find(fd); 256   4100 auto it = registered_descs_.find(fd);
HITCBC 257   4751 if (it == registered_descs_.end()) 257   4100 if (it == registered_descs_.end())
MISUBC 258   ✗ return; 258   ✗ return;
259   259  
HITCBC 260   4751 registered_descs_.erase(it); 260   4100 registered_descs_.erase(it);
261   261  
HITCBC 262   4751 if (fd == max_fd_) 262   4100 if (fd == max_fd_)
263   { 263   {
HITCBC 264   4417 max_fd_ = pipe_fds_[0]; 264   3765 max_fd_ = pipe_fds_[0];
HITCBC 265   8428 for (auto& [registered_fd, state] : registered_descs_) 265   7124 for (auto& [registered_fd, state] : registered_descs_)
266   { 266   {
HITCBC 267   4011 if (registered_fd > max_fd_) 267   3359 if (registered_fd > max_fd_)
HITCBC 268   3918 max_fd_ = registered_fd; 268   3266 max_fd_ = registered_fd;
269   } 269   }
270   } 270   }
HITCBC 271   4751 } 271   4100 }
272   272  
273   inline void 273   inline void
HITCBC 274   2134 select_scheduler::notify_reactor() const 274   1809 select_scheduler::notify_reactor() const
275   { 275   {
HITCBC 276   2134 interrupt_reactor(); 276   1809 interrupt_reactor();
HITCBC 277   2134 } 277   1809 }
278   278  
279   inline void 279   inline void
HITCBC 280   12123 select_scheduler::interrupt_reactor() const 280   10841 select_scheduler::interrupt_reactor() const
281   { 281   {
HITCBC 282   12123 char byte = 1; 282   10841 char byte = 1;
HITCBC 283   12123 [[maybe_unused]] auto r = ::write(pipe_fds_[1], &byte, 1); 283   10841 [[maybe_unused]] auto r = ::write(pipe_fds_[1], &byte, 1);
HITCBC 284   12123 } 284   10841 }
285   285  
286   inline long 286   inline long
HITCBC 287   244600 select_scheduler::calculate_timeout(long requested_timeout_us) const 287   132022 select_scheduler::calculate_timeout(long requested_timeout_us) const
288   { 288   {
HITCBC 289   244600 if (requested_timeout_us == 0) 289   132022 if (requested_timeout_us == 0)
290   return 0; // LCOV_EXCL_LINE run_task passes 0 via task_interrupted_, never through this argument 290   return 0; // LCOV_EXCL_LINE run_task passes 0 via task_interrupted_, never through this argument
291   291  
HITCBC 292   244600 auto nearest = timer_svc_->nearest_expiry(); 292   132022 auto nearest = timer_svc_->nearest_expiry();
HITCBC 293   244600 if (nearest == timer_service::time_point::max()) 293   132022 if (nearest == timer_service::time_point::max())
HITCBC 294   961 return requested_timeout_us; 294   1091 return requested_timeout_us;
295   295  
HITCBC 296   243639 auto now = std::chrono::steady_clock::now(); 296   130931 auto now = std::chrono::steady_clock::now();
HITCBC 297   243639 if (nearest <= now) 297   130931 if (nearest <= now)
HITCBC 298   694 return 0; 298   484 return 0;
299   299  
300   auto timer_timeout_us = 300   auto timer_timeout_us =
HITCBC 301   242945 std::chrono::duration_cast<std::chrono::microseconds>(nearest - now) 301   130447 std::chrono::duration_cast<std::chrono::microseconds>(nearest - now)
HITCBC 302   242945 .count(); 302   130447 .count();
303   303  
HITCBC 304   242945 constexpr auto long_max = 304   130447 constexpr auto long_max =
305   static_cast<long long>((std::numeric_limits<long>::max)()); 305   static_cast<long long>((std::numeric_limits<long>::max)());
306   auto capped_timer_us = 306   auto capped_timer_us =
HITCBC 307   242945 (std::min)((std::max)(static_cast<long long>(timer_timeout_us), 307   130447 (std::min)((std::max)(static_cast<long long>(timer_timeout_us),
HITCBC 308   242945 static_cast<long long>(0)), 308   130447 static_cast<long long>(0)),
HITCBC 309   242945 long_max); 309   130447 long_max);
310   310  
HITCBC 311   242945 if (requested_timeout_us < 0) 311   130447 if (requested_timeout_us < 0)
HITCBC 312   242943 return static_cast<long>(capped_timer_us); 312   130445 return static_cast<long>(capped_timer_us);
313   313  
314   return static_cast<long>( 314   return static_cast<long>(
HITCBC 315   2 (std::min)(static_cast<long long>(requested_timeout_us), 315   2 (std::min)(static_cast<long long>(requested_timeout_us),
HITCBC 316   2 capped_timer_us)); 316   2 capped_timer_us));
317   } 317   }
318   318  
319   inline void 319   inline void
HITCBC 320   269187 select_scheduler::run_task(lock_type& lock, context_type& ctx, long timeout_us) 320   142811 select_scheduler::run_task(lock_type& lock, context_type& ctx, long timeout_us)
321   { 321   {
322   long effective_timeout_us = 322   long effective_timeout_us =
HITCBC 323   269187 task_interrupted_ ? 0 : calculate_timeout(timeout_us); 323   142811 task_interrupted_ ? 0 : calculate_timeout(timeout_us);
324   324  
325   // Snapshot registered descriptors while holding lock. 325   // Snapshot registered descriptors while holding lock.
326   // Record which fds need write monitoring to avoid a hot loop: 326   // Record which fds need write monitoring to avoid a hot loop:
327   // select is level-triggered so writable sockets (nearly always 327   // select is level-triggered so writable sockets (nearly always
328   // writable) would cause select() to return immediately every 328   // writable) would cause select() to return immediately every
329   // iteration if unconditionally added to write_fds. Membership 329   // iteration if unconditionally added to write_fds. Membership
330   // stays opt-in: a parked write wait opts in the same way a 330   // stays opt-in: a parked write wait opts in the same way a
331   // parked write or connect op does. 331   // parked write or connect op does.
332   struct fd_entry 332   struct fd_entry
333   { 333   {
334   int fd; 334   int fd;
335   reactor_descriptor_state* desc; 335   reactor_descriptor_state* desc;
336   bool needs_write; 336   bool needs_write;
337   }; 337   };
338   fd_entry snapshot[FD_SETSIZE]; 338   fd_entry snapshot[FD_SETSIZE];
HITCBC 339   269187 int snapshot_count = 0; 339   142811 int snapshot_count = 0;
340   340  
HITCBC 341   718487 for (auto& [fd, desc] : registered_descs_) 341   356310 for (auto& [fd, desc] : registered_descs_)
342   { 342   {
HITCBC 343   449300 if (snapshot_count < FD_SETSIZE) 343   213499 if (snapshot_count < FD_SETSIZE)
344   { 344   {
HITCBC 345   449300 conditionally_enabled_mutex::scoped_lock desc_lock(desc->mutex); 345   213499 conditionally_enabled_mutex::scoped_lock desc_lock(desc->mutex);
HITCBC 346   449300 snapshot[snapshot_count].fd = fd; 346   213499 snapshot[snapshot_count].fd = fd;
HITCBC 347   449300 snapshot[snapshot_count].desc = desc; 347   213499 snapshot[snapshot_count].desc = desc;
HITCBC 348   449300 snapshot[snapshot_count].needs_write = 348   213499 snapshot[snapshot_count].needs_write =
HITCBC 349   449300 (desc->write_op || desc->connect_op || desc->wait_write_op); 349   213499 (desc->write_op || desc->connect_op || desc->wait_write_op);
HITCBC 350   449300 ++snapshot_count; 350   213499 ++snapshot_count;
HITCBC 351   449300 } 351   213499 }
352   } 352   }
353   353  
HITCBC 354   269187 if (lock.owns_lock()) 354   142811 if (lock.owns_lock())
HITCBC 355   244601 lock.unlock(); 355   132023 lock.unlock();
356   356  
HITCBC 357   269187 task_cleanup on_exit{this, &lock, ctx}; 357   142811 task_cleanup on_exit{this, &lock, ctx};
358   358  
359   fd_set read_fds, write_fds, except_fds; 359   fd_set read_fds, write_fds, except_fds;
HITCBC 360   4576179 FD_ZERO(&read_fds); 360   2427787 FD_ZERO(&read_fds);
HITCBC 361   4576179 FD_ZERO(&write_fds); 361   2427787 FD_ZERO(&write_fds);
HITCBC 362   4576179 FD_ZERO(&except_fds); 362   2427787 FD_ZERO(&except_fds);
363   363  
HITCBC 364   269187 FD_SET(pipe_fds_[0], &read_fds); 364   142811 FD_SET(pipe_fds_[0], &read_fds);
HITCBC 365   269187 int nfds = pipe_fds_[0]; 365   142811 int nfds = pipe_fds_[0];
366   366  
HITCBC 367   718487 for (int i = 0; i < snapshot_count; ++i) 367   356310 for (int i = 0; i < snapshot_count; ++i)
368   { 368   {
HITCBC 369   449300 int fd = snapshot[i].fd; 369   213499 int fd = snapshot[i].fd;
HITCBC 370   449300 FD_SET(fd, &read_fds); 370   213499 FD_SET(fd, &read_fds);
HITCBC 371   449300 if (snapshot[i].needs_write) 371   213499 if (snapshot[i].needs_write)
HITCBC 372   7385 FD_SET(fd, &write_fds); 372   5204 FD_SET(fd, &write_fds);
HITCBC 373   449300 FD_SET(fd, &except_fds); 373   213499 FD_SET(fd, &except_fds);
HITCBC 374   449300 if (fd > nfds) 374   213499 if (fd > nfds)
HITCBC 375   268593 nfds = fd; 375   142125 nfds = fd;
376   } 376   }
377   377  
378   struct timeval tv; 378   struct timeval tv;
HITCBC 379   269187 struct timeval* tv_ptr = nullptr; 379   142811 struct timeval* tv_ptr = nullptr;
HITCBC 380   269187 if (effective_timeout_us >= 0) 380   142811 if (effective_timeout_us >= 0)
381   { 381   {
HITCBC 382   268428 tv.tv_sec = effective_timeout_us / 1000000; 382   141994 tv.tv_sec = effective_timeout_us / 1000000;
HITCBC 383   268428 tv.tv_usec = effective_timeout_us % 1000000; 383   141994 tv.tv_usec = effective_timeout_us % 1000000;
HITCBC 384   268428 tv_ptr = &tv; 384   141994 tv_ptr = &tv;
385   } 385   }
386   386  
HITCBC 387   269187 int ready = ::select(nfds + 1, &read_fds, &write_fds, &except_fds, tv_ptr); 387   142811 int ready = ::select(nfds + 1, &read_fds, &write_fds, &except_fds, tv_ptr);
388   388  
389   // EINTR: signal interrupted select(), just retry. 389   // EINTR: signal interrupted select(), just retry.
390   // EBADF: an fd was closed between snapshot and select(); retry 390   // EBADF: an fd was closed between snapshot and select(); retry
391   // with a fresh snapshot from registered_descs_. 391   // with a fresh snapshot from registered_descs_.
392   // Both fall through with no ready descriptors rather than 392   // Both fall through with no ready descriptors rather than
393   // returning: the caller handed this function an owned lock that 393   // returning: the caller handed this function an owned lock that
394   // only the epilogue below re-acquires. 394   // only the epilogue below re-acquires.
HITCBC 395   269187 if (ready < 0) 395   142811 if (ready < 0)
396   { 396   {
HITCBC 397   3 if (errno != EINTR && errno != EBADF) 397   3 if (errno != EINTR && errno != EBADF)
HITCBC 398   1 detail::throw_system_error(make_err(errno), "select"); 398   1 detail::throw_system_error(make_err(errno), "select");
HITCBC 399   2 ready = 0; 399   2 ready = 0;
400   } 400   }
401   401  
402   // Process timers outside the lock 402   // Process timers outside the lock
HITCBC 403   269186 timer_svc_->process_expired(); 403   142810 timer_svc_->process_expired();
404   404  
HITCBC 405   269186 ready_queue local_ops; 405   142810 ready_queue local_ops;
406   406  
HITCBC 407   269186 if (ready > 0) 407   142810 if (ready > 0)
408   { 408   {
HITCBC 409   252925 if (FD_ISSET(pipe_fds_[0], &read_fds)) 409   135167 if (FD_ISSET(pipe_fds_[0], &read_fds))
410   { 410   {
411   char buf[256]; 411   char buf[256];
HITCBC 412   11340 while (::read(pipe_fds_[0], buf, sizeof(buf)) > 0) 412   10116 while (::read(pipe_fds_[0], buf, sizeof(buf)) > 0)
413   { 413   {
414   } 414   }
415   } 415   }
416   416  
HITCBC 417   655011 for (int i = 0; i < snapshot_count; ++i) 417   333947 for (int i = 0; i < snapshot_count; ++i)
418   { 418   {
HITCBC 419   402086 int fd = snapshot[i].fd; 419   198780 int fd = snapshot[i].fd;
HITCBC 420   402086 reactor_descriptor_state* desc = snapshot[i].desc; 420   198780 reactor_descriptor_state* desc = snapshot[i].desc;
421   421  
HITCBC 422   402086 std::uint32_t flags = 0; 422   198780 std::uint32_t flags = 0;
HITCBC 423   402086 if (FD_ISSET(fd, &read_fds)) 423   198780 if (FD_ISSET(fd, &read_fds))
HITCBC 424   251839 flags |= reactor_event_read; 424   132628 flags |= reactor_event_read;
HITCBC 425   402086 if (FD_ISSET(fd, &write_fds)) 425   198780 if (FD_ISSET(fd, &write_fds))
HITCBC 426   2126 flags |= reactor_event_write; 426   1801 flags |= reactor_event_write;
HITCBC 427   402086 if (FD_ISSET(fd, &except_fds)) 427   198780 if (FD_ISSET(fd, &except_fds))
HITCBC 428   16 flags |= reactor_event_error; 428   16 flags |= reactor_event_error;
429   429  
HITCBC 430   402086 if (flags == 0) 430   198780 if (flags == 0)
HITCBC 431   148129 continue; 431   64360 continue;
432   432  
HITCBC 433   253957 desc->add_ready_events(flags); 433   134420 desc->add_ready_events(flags);
434   434  
HITCBC 435   253957 bool expected = false; 435   134420 bool expected = false;
HITCBC 436   253957 if (desc->is_enqueued_.compare_exchange_strong( 436   134420 if (desc->is_enqueued_.compare_exchange_strong(
437   expected, true, std::memory_order_release, 437   expected, true, std::memory_order_release,
438   std::memory_order_relaxed)) 438   std::memory_order_relaxed))
439   { 439   {
HITCBC 440   253957 local_ops.push(desc); 440   134420 local_ops.push(desc);
441   } 441   }
442   } 442   }
443   } 443   }
444   444  
HITCBC 445   269186 lock.lock(); 445   142810 lock.lock();
446   446  
HITCBC 447   269186 completed_ops_.splice(local_ops); 447   142810 completed_ops_.splice(local_ops);
HITCBC 448   269187 } 448   142811 }
449   449  
450   } // namespace boost::corosio::detail 450   } // namespace boost::corosio::detail
451   451  
452   #endif // BOOST_COROSIO_HAS_SELECT 452   #endif // BOOST_COROSIO_HAS_SELECT
453   453  
454   #endif // BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP 454   #endif // BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP